The piezoresistive transducer has been widely employed in the measurement of pressure in various types of environments and application. Most semiconductor strain gage transducers employed in the prior art have used the longitudinal piezoresistive effect.
Generally speaking, a great majority of pressure transducers employ a relatively thin diaphragm which may be fabricated from a semiconductor material such as silicon. This diaphragm has deposited or diffused thereon, a piezoresistive strain gage configuration such as a bridge circuit wherein the resistors associated with the bridge exhibit a change in resistance according to the deflection of the diaphragm. Accordingly, one can then monitor the bridge circuit to obtain an electrical current indicative of the applied pressure or force. Hence, the prior art is replete with a great number of patents and literature directed towards such embodiments.
In any event, it becomes extremely difficult to employ the longitudinal piezoresistive effect in the measurement of extremely high pressures or pressures in excess of 25,000 psi. The prior art has been cognizant of the transverse or shear piezoresistive effect and many articles and publications exist which essentially describe the theory of operation of the same. For example, see an article entitled MONOGRAPH 4056 by the Bell Telephone System publication entitled SEMICONDUCTING STRESS TRANSDUCERS USING TRANSVERSE AND SHEAR PIEZORESISTANCE by W. G. Pfann and R. N. Thurston.
As indicated in the above noted article and others, there have been transducers fabricated which make use of both the transverse and longitudinal piezoresistive coefficients. In a transverse situation, the tensile or compresssive stress is normal to the current flow through the sensors as compared to the longitudinal situation, where the tensile and compressive stress are in the same direction as the current flow through the sensors. Hence, by optimum design, one can faithfully transmit transverse and shear strains to semiconductor sensor devices.
In any event, the use of a conventional type of diaphragm in such situations is, as indicated, extremely difficult as the diaphragm does not behave according to the constraints determined by low pressure measurements. The prior art has attempted to formulate a high pressure transducer employing the transverse effect by using a thin disk of silicon with suitable sensors positioned between two compression members. Such techniques, however, are inherent with other problems such as thermal stability and in general, temperature fluctuations. As one can ascertain, a high pressure environment can be associated with high temperature and hence, the nature of the operating environment imposes severe restrictions on the type of device used.
Apart from the above noted considerations, there is a further problem in directing leads from the transducer structure to enable the monitoring of the bridge characteristics in that environment.
It is therefore an object of the present invention to describe a high pressure transducer structure which is particularly adapted for the measurement of pressures in excess of 25,000 psi while providing thermal stability employing the transverse piezoresistive effect.